I used Monogame back when it was a proprietary framework called XNA developed at Microsoft.
You used to be able to use XNA to build Indie games for the Xbox 360, hard to believe, but this is going on 15 years ago at this point.
I built two indie games and made a couple of hundred bucks back when I was in High School. It's actually what got me into programming in the first place.
I'm happy to see that XNA became Monogame, it's one of the best frameworks I've ever used for gamedev.
Same here (minus the making any money). XNA is how I started learning graphics programming and started my interest in things like physics engines. Shawn Hargreaves had great blog posts on gamedev back then, too.